Meltblown fibers are fibers formed by extruding a molten thermoplastic material through a plurality of fine, usually circular, die capillaries as molten threads or filaments into converging, usually hot and high velocity, gas (e.g. air) streams to attenuate the filaments of molten thermoplastic material and form fibers. During the meltblowing process, the diameter of the molten filaments are reduced by the drawing air to a desired size. Thereafter, the meltblown fibers are carried by the high velocity gas stream and are deposited on a collecting surface to form a web of randomly disbursed meltblown fibers. Such a process is disclosed, for example, in U.S. Pat. Nos. 3,849,241 to Buntin et al., 4,526,733 to Lau, and 5,160,746 to Dodge, II et al., all of which are hereby incorporated herein by this reference. Meltblown fibers may be continuous or discontinuous and are generally smaller than ten microns in average diameter.
In a conventional meltblowing process, molten polymer is provided to a die that is disposed between a pair of air plates that form a primary air nozzle. Standard meltblown equipment includes a die tip with a single row of capillaries along a knife edge. Typical die tips have approximately 30 capillary exit holes per linear inch of die width. The die tip is typically a 60.degree. wedge-shaped block converging at the knife edge at the point where the capillaries are located. The air plates in many known meltblowing nozzles are mounted in a recessed configuration such that the tip of the die is set back from the primary air nozzle. However, air plates in some nozzles are mounted in a flush configuration where the air plate ends are in the same horizontal plane as the die tip; in other nozzles the die tip is in a protruding or "stick-out" configuration so that the tip of the die extends past the ends of the air plates. Moreover, as disclosed in U.S. Pat. No. 5,160,746 to Dodge II et al, more than one air flow stream can be provided for use in the nozzle.
In most known configurations of meltblowing nozzles, hot air is provided through the primary air nozzle formed on each side of the die tip. The hot air heats the die and thus prevents the die from freezing as the molten polymer exits and cools. In this way the die is prevented from becoming clogged with solidifying polymer. The hot air also draws, or attenuates, the melt into fibers.
Primary hot air flow rates typically range from about 20 to 24 standard cubic feet per minute per inch of die width (scfm/in).
Primary air pressure typically ranges from 5 to 10 pounds per square inch gauge (psig). Primary air temperature typically ranges from 450.degree. to 600.degree. Fahrenheit (F), but temperatures of 750.degree. F. are not uncommon. The particular temperature of the primary hot air flow will depend on the particular polymer being drawn as well as other characteristics desired in the meltblown web.
Expressed in terms of the amount of polymer material flowing per inch of the die per unit of time, polymer throughput is typically 0.5 to 1.25 grams per hole per minute (ghm). Thus, for a die having 30 holes per inch, polymer throughput is typically about 2 to 5 lbs/inch/hour (PIH).
Moreover, in order to form meltblown fibers from an input of about five pounds per inch per hour of the polymer melt, about one hundred pounds per inch per hour of hot air is required to draw or attenuate the melt into discrete fibers. This drawing air must be heated to a temperature on the order of 400-600.degree. F. in order to maintain proper heat to the die tip.
Because such high temperatures must be used, a substantial amount of heat must be removed from the fibers in order to quench (or solidify) the fibers leaving the die orifice. Cold gases, such as air, have been used to accelerate cooling and solidification of the meltblown fibers. In particular, in U.S. Pat. No. 5,075,068 to Milligan et al and U.S. Pat. No. 5,080,569 to Gubernick et al, which are hereby incorporated herein by reference, secondary air flowing in a cross-flow perpendicular, or 90.degree., direction relative to the direction of fiber elongation, has been used to quench meltblown fibers and produce smaller diameter fibers. In addition, U.S. Pat. No. 5,607,701 to Allen et al, which is hereby incorporated herein by reference, uses a cooler pressurized quench air that fills chamber 71 and results in faster cooling and solidification of the fibers. In U.S. Pat. No. 4,112,159 to Pall, which is hereby incorporated herein by reference, a cold air flow is used to attenuate the fibers when it is desired to decrease the attenuation of the fibers.
Generally, in a typical meltblown process, the energy in the primary air is approximately seven times that required for the polymer stream, and the temperature range of the hot primary air will generally be 500.degree. F. to 550.degree. F. The conventional meltblown process has high energy demands because the polymer must be heated, the die tip must be kept heated, hot primary air flow attenuates the polymer to the desired diameter, and the hot attenuated fibers must be quenched. Thus, the meltblowing process is energy intensive, both in adding heat during fiber formation and in removing heat during fiber quenching.
Through the control of air and die tip temperatures, air pressure, and polymer feed rate, the diameter of the fiber formed during the meltblown process may be regulated. For example, typical meltblown polypropylene fibers have a diameter of 3 to 4 microns.
After cooling, the fibers are collected to form an integrated web. In particular, the fibers are collected on a forming web that comprises a moving mesh screen or belt located below the die tip. In order to provide enough space beneath the die tip for fiber forming, attenuation and cooling, forming distances of at least about 8 to 12 inches between the polymer die tip and the top of the mesh screen are required in the typical meltblowing process.
However, forming distances as low as 4 inches are described in U.S. Pat. No. 4,526,733 to Lau (hereafter the '733 patent), which is hereby incorporated herein by reference. As described in Example 3 of the '733 patent, the shorter forming distances are achieved with attenuating air flows of at least 100.degree. F. cooler than the temperature of the molten polymer. For example, Lau discloses the use of attenuating air at 150.degree. F. for polypropylene melt at a temperature of 511.degree. F. to allow a forming distance between die tip and forming belt of 4 inches. The Lau patent incorporates passive air gaps 36 (shown in FIG. 4 of Lau) to insulate the die tip.
Zebra striping is a problem that sometimes results during high speed meltblowing line applications. Stripes may form in the cross machine direction, resulting from the flapping of the turbulent air jet. If the forming distance between the die and the forming fabrics can be decreased, Zebra striping will likewise be decreased. Uniformity will also improve due to the smaller scale turbulent structure in the primary air jet at low forming distances. As the forming distance increases, the primary air jet decays more and the formation becomes worse.